Thin-walled metallic cans, such as those used for packaging beer, soft drinks and other beverages, are currently produced in quantities exceeding ninety billion cans per year in the United States. Because of this extremely high volume of production, even the smallest savings in the metal from these cans are made can result in enormous cost savings. It is therefore meaningful to reduce the starting gauge of the metal used to make such cans by as little as one one-tenthousandth of an inch (0.0001"). Current technologies allow the production of 12 ounce cans having side wall thicknesses as low as 0.005" without loss of integrity because, structurally, the sealed can is a cylindrical "pressure vessel." That is, it relies for part of its strength on the internal force exerted by the liquid and gas contained within the can. In contrast, the bottom of conventional cans continues to be manufactured with a thickness of about 0.010" to about 0.011" in order to withstand the axial loads of up to 200 pounds imposed on unsealed cans during manufacturing and filling operations and also to resist unwanted deformation of the sealed cans from axial loads caused by shipping or stacking or from internal pressures, which may range from 40 psig up to over 100 psig.
Most applications for metallic beverage cans have additional requirements for stand stability, stacking stability, mobility and resistance to shipping and handling loads and vibrations.
Stand stability relates to a can's ability to rest in an upright position on a flat horizontal surface without wobbling or tipping. Stand stability is important during the automated processing of both empty and filled cans as well as for consumer convenience and acceptance. The features on the can bottom which support an upright can on a flat horizontal surface are known as "stand features." The diameter of an imaginary circle centered on the longitudinal axis of the can and passing through the stand features represents a parameter called the "stand diameter." Stand stability is increased by providing stand features which are disposed radially outwardly as far as possible from the can's longitudinal axis, i.e., by increasing the stand diameter.
Stacking stability relates to a can's ability to rest stably in an upright position on the top of a below adjacent can. Stacking stability includes resistance to tipping or wobbling by the can as well as resistance to lateral movement between the stacked cans. Stacking stability is typically achieved by providing features in the bottom profile of the upper can which interfit with features in the lid profile of the lower can and by providing sufficient clearance between the bottom of the upper can and the lid and tab of the lower can.
Mobility relates to a can's ability to transit automated handling and conveying equipment without tipping, catching, jamming or otherwise impeding operations. For example, cans must be able to transit the "dead plates" in a conveyor system without tipping over or catching. Mobility is of particular concern for empty cans because their light weight reduces their resistance to tipping, however mobility is necessary for both empty and filled cans. It is known that mobility is affected by both stand diameter and by the profile of the stand features, i.e., increasing stand diameter typically increases mobility and increasing the radius of the stand features typically increases mobility.
Resistance to shipping and handling loads and vibrations relates to a can's ability to withstand the axial loads imposedby having additional cans stacked above during shipping and by the vibrations associated with transportation in trucks and other distribution and delivery vehicles. Vibrations and axial loads combine to produce flexures in the can walls which may ultimately lead to fatigue cracking of the metal. The interior lid panel and interior bottom wall of the can are the most susceptible to such flexure-induced cracking. It is therefore preferable that cans in stacking engagement have no contact between the interior bottom wall of the above-adjacent can and the interior lid panel or pull tab of the below-adjacent can.
To meet the structural requirements for can bottoms, conventional industry practice is to form the can bottom into an externally concave, i.e., upwardly domed shape that will not interfere with stand stability if it bulges outward somewhat under internal pressure and will not contact the interior lid panel or lifting tab of another can when in stacked engagement. However, such upwardly domed bottoms must be formed of relatively thick material to resist excessive deformation. In addition, upwardly domed bottom walls reduce the internal volume of the can and may experience a failure mode known as "dome reversal" if the internal pressure becomes too high, thus rendering the can unstable and thus unsalable.
U.S. Pat. Nos. 3,904,069, 4,412,627 and 4,431,112 contain discussions of upwardly domed can bottoms and the phenomena of dome reversal caused by internal pressure. Upwardly domed can bottoms will not be discussed further herein, however, since the present invention does not employ an upwardly domed can bottom and is intended to be an alternative to that approach.
An alternative to can designs having a conventional upwardly domed bottom wall is found in the "displaceable" bottom wall designs of U.S. Pat. Nos. 3,979,009, 4,037,752 and 5,421,480. Displaceable bottom wall designs have first stand features which provide stand stability when the can is unpressurized, however, as the internal pressure in the can exceeds a predetermined level, the bottom wall is displaced downwardly to provide second stand features which replace the first features in providing stand stability. Such displaceable bottom wall designs experience a change in the overall height of the can when the bottom wall is displaced outwardly. Displaceable can bottoms will not be discussed further herein, however, since the present invention does not employ a displaceable bottom wall design and is intended to be an alternative to that approach.
It is an object of the present invention to reduce the thickness of the metal in a can bottom wall without affecting the structural integrity of the can. Another object of the invention is to reduce the thickness of the can bottom wall to less than about 0.010" while still enabling the unsealed can to withstand an axial force of about 200 pounds without permanent deformation. A further object of the current invention is to provide a can having an externally convex, i.e., downwardly domed bottom wall which minimizes the "growth", or increase in overall height of the sealed can when it is subjected to a range of internal pressures. A further object of the current invention is to provide a can which exhibits stand stability, stacking stability and mobility even when subjected to a range of internal pressures. It is yet another object of the current invention to provide a can having a downwardly domed bottom wall which, when placed in stacking engagement with a below adjacent can, does not contact the interior lid panel or pull-tab of the can below when subjected to a range of internal pressures and vibrations. It is still another object of the current invention to provide a can with a bottom wall formed with primarily outwardly convex mechanical features. It is still another object of the current invention to provide a can with a bottom wall which does not undergo a change in mechanical modes when the sealed can is subjected to a range of internal pressures.